Polyethylene terephthalate is one of the most important engineering thermal plastics used for fibers, films and bottles. Despite its wide applications and vast global market, PET has shortcomings, which limits it usage in many areas. PET has a glass transition temperature (Tg) of 80 DEGREE Celsius, this temperature is too low for certain applications. Increase in glass transition temperature, high temperature mechanical properties, and dimensional stability is of great importance to further expand the applications of PET. Significant research efforts have been made toward this goal, using a variety of approaches. In this work, we attempt to improve the properties of PET melt spun filament. Three strategies has been investigated (i) copolymerization of more rigid comonomer, 4, 4' bibenzoate unit into the PET structure, (ii) UV crosslinking of functionalized PET fiber, and (iii) Reinforcing PET matrix with carbon nanofibers.

This dissertation discusses the polymorphism, crystallization and melting behavior of propylene-ethylene random copolymers. It also discusses the results of studies of the miscibility and the competitive liquid-liquid demixing and crystallization processes in blends of poly(propylene) and poly(1-butene). In the first part of this study, polymorphism of propylene-ethylene copolymers is studied by wide angle X-ray diffraction. By comparing the a and y crystal phase contents in samples with different ethylene content as well as samples isothermally crystallized at different temperatures, it was shown that increasing ethylene content as well as increasing crystallization temperature promotes the formation of the y-phase. Comparison of the results from fractionated samples and unfractionated samples with similar ethylene contents reveals that in propylene-ethylene copolymers with similar micro-structure, the polymorphism, crystallization and melting behavior are mainly determined by their ethylene content. The issue of co-unit inclusion and its effect on crystallization and melting behavior are also discussed. In the second part of this dissertation, the miscibility behavior of atactic - poly(propylene) (at-PP) and atactic poly(1-butene) (ai-P1B) with different molecular weights is investigated by differential scanning calorimetry. The phase diagram of at-PP and at-P1B blend of molecular weight (87K/48.5K) shows a upper critical solution temperature (UCST) behavior. The UCST behavior is consistent with predictions by the group contribution method. Miscibility behavior of high molecular weight isotactic poly(propylene) (it-PP) and isotactic poly(1-butene) (it-P1B) blend is investigated by a combination of optical microscopy and scanning electron microscopy, differential scanning calorimetry and dynamic mechanical analysis. These studies reveal that for the molecular weights investigated, it-PP and it-P1B form blends that are partially miscible in the liquid state. Liquid-liquid demixing is observed by optical microscopy at temperatures above the melting temperature of the it-PP component and is also inferred from scanning electron micrographs of the freeze fracture surface of quenched blends after extraction of the it- P1B component with cyclohexane. It-PP spherulites grow through both liquid phases at relative rates that depend markedly on the crystallization temperature. The complex multiple-melting behavior of the it-PP component in the blend is explained in terms of a bimodal distribution of it-PP lamellar crystals which result from crystal growth in the phase-separated liquid. Finally, the dynamic mechanical analysis data are explained in terms of a liquid-liquid demixing process that results in a significant degree of phase mixing. / Ph. D.